2.4 performance reduction on SORT

19 Oct 2026


      Hi,
I’m preparing the migration from an old Rocky 8 with Dovecot 2.3 to
Rocky 9 with Dovecot 2.4.5.
I’m using Apache Solr as the FTS driver for the messages and Maildir as
storage. Everything works great on 2.3, but since I started
experimenting on 2.4 I noticed a big performance reduction on FTS
queries in IMAP, specifically on the virtual all mailbox.
I’ve noticed that in Dovecot 2.4 the SORT command is taking 100x more
than v2.3 to complete. Running the same command (SORT (REVERSE DATE)
US-ASCII BODY "Hi") on a medium sized mailbox (about 8GB) on Dovecot 2.3
it completes in seconds and on Dovecot 2.4 it takes about 3-5 minutes.
On both cases the query returns about 80k messages.
To exclude Solr as a culprit, using a network sniff tool, I checked that
when Dovecot queries Solr for the FTS, it responds within seconds (as
the SEARCH command does) so most of the execution time is spent on
actually sorting the messages.
Is this a regression? Am I missing some cache optimization?
For context, here is my Dovecot 2.3 config:
2.3.21.1 (d492236fa0): /etc/dovecot/dovecot.conf
Pigeonhole version 0.5.21.1 (49005e73)
OS: Linux 4.18.0-553.137.1.el8_10.x86_64 x86_64 Rocky Linux release
8.10 (Green Obsidian)
mail_attribute_dict = file:%h/Maildir/dovecot-attributes
mail_fsync = always
mail_location = maildir:~/Maildir:VOLATILEDIR=/dev/shm/dovecot/%2.256Nu/%u
mail_plugins = quota quota_clone acl zlib fts fts_solr virtual notify
mail_log
maildir_very_dirty_syncs = yes
mmap_disable = yes
namespace Virtual {
hidden = yes
list = no
location = virtual:/etc/dovecot/virtual:INDEX=~/Maildir/virtual
prefix = Virtual/
separator = /
subscriptions = no
}
namespace inbox {
inbox = yes
location =
mailbox Archive {
auto = subscribe
special_use = \Archive
}
mailbox Drafts {
auto = subscribe
special_use = \Drafts
}
mailbox Sent {
auto = subscribe
special_use = \Sent
}
mailbox "Sent Messages" {
special_use = \Sent
}
mailbox Spam {
auto = subscribe
special_use = \Junk
}
mailbox Trash {
auto = subscribe
special_use = \Trash
}
mailbox virtual/All {
comment = All my messages
special_use = \All
}
prefix =
separator = /
}
plugin {
acl = vfile
acl_shared_dict = proxy::acl
fts = solr
fts_autoindex = no
fts_autoindex_exclude = Drafts
fts_autoindex_exclude2 = Spam
fts_autoindex_exclude3 = Trash
fts_autoindex_exclude4 = Sent
fts_autoindex_max_recent_msgs = 500
fts_decoder = decode2text
fts_header_excludes = Received Authentication-Results DKIM-* ARC-*
X-* Feedback-* List-*
fts_header_includes = From To Cc Bcc Subject Message-ID X-Identifier
fts_index_timeout = 8s
fts_solr = url=.../solr/dovecot/
zlib_save = gz
zlib_save_level = 6
}
service decode2text {
executable = script /usr/libexec/dovecot/decode2text.sh
unix_listener decode2text {
mode = 0666
}
user = dovecot
}
service imap {
executable = imap imap-postlogin
process_limit = 8000
vsz_limit = 2 G
}
Dovecot 2.4 config:
2.4.5-2 (0cbb641e3e): /etc/dovecot/dovecot.conf
Pigeonhole version 2.4.5-2 (742bb892)
OS: Linux 5.14.0-687.36.1.el9_8.x86_64 x86_64 Rocky Linux release 9.8
(Blue Onyx)
4 default setting changes since version 2.4.4
dovecot_config_version = 2.4.4
acl_dict_index = yes
acl_driver = vfile
dovecot_storage_version = 2.4.4
fts_header_excludes {
Received = yes
Authentication-Results = yes
DKIM-* = yes
ARC-* = yes
X-* = yes
Feedback-* = yes
List-* = yes
}
fts_header_includes {
From = yes
To = yes
Cc = yes
Bcc = yes
Subject = yes
Message-ID = yes
X-Identifier = yes
}
fts_search_timeout = 8s
mail_compress_write_method = gz
mail_driver = maildir
mail_fsync = always
mail_gid = 89
mail_path = ~/Maildir
mail_plugins = acl mail_compress virtual fts fts_solr notify quota
quota_clone
mail_uid = 89
mail_volatile_path = /dev/shm/dovecot/%{user | md5 | substr(0, 8) % 256
| hex(2)}/%{user}
mmap_disable = yes
verbose_proctitle = yes
mail_attribute {
dict file {
path = %{home}/Maildir/dovecot-attributes
}
}
maildir {
very_dirty_syncs = yes
}
namespace inbox {
inbox = yes
separator = /
mailbox Drafts {
auto = subscribe
special_use = "\\Drafts"
}
mailbox Spam {
auto = subscribe
autoexpunge = 31 days
special_use = "\\Junk"
}
mailbox Junk {
auto = no
}
mailbox Trash {
auto = subscribe
autoexpunge = 31 days
special_use = "\\Trash"
quota_storage_extra = 100M
}
mailbox Sent {
auto = subscribe
special_use = "\\Sent"
}
mailbox Archive {
auto = subscribe
special_use = "\\Archive"
}
mailbox "Sent Messages" {
special_use = "\\Sent"
}
}
namespace Virtual {
mail_cache_path = ~/Maildir/virtual
mail_driver = virtual
mail_index_path = ~/Maildir/virtual
mail_path = /var/lib/dovecot/virtual
hidden = yes
list = no
prefix = Virtual/
separator = /
subscriptions = no
mailbox All {
comment = All my messages
special_use = "\\All"
}
}
protocol imap {
mail_plugins {
imap_acl = yes
last_login = yes
imap_quota = yes
virtual = yes
}
}
fts solr {
soft_commit = yes
}
mailbox Drafts {
fts_autoindex = no
}
mailbox Spam {
fts_autoindex = no
}
mailbox Trash {
fts_autoindex = no
}
mailbox Sent {
fts_autoindex = no
}
language en {
default = yes
}
language it {
}
Thanks
--
Gioele Pannetto